The present invention relates to apparatus and a method for detecting both the spatial and temporal behavior of a fetus.
It is useful to be able to quantify the degree of movement of a fetus within the uterus as this can provide information regarding the health of the fetus during the progress of pregnancy. In addition, knowing the current position and presentation of the fetus within the uterus can provide information regarding presentation of the fetus. Currently there are several techniques applied to antepartum fetal surveillance in order to assess fetal health. These techniques include:
Contraction Stress Test
Nonstress Test
Fetal movement assessment
Biophysical Profile
Modified Biophysical Profile
Umbilical Artery Doppler Velocimetry
The Contraction Stress Test (CST) uses Doppler Ultrasound to measure fetal heart rate and is based on the response of the fetal heart rate to uterine contractions. The technique works on the principle that fetal oxygenation will momentarily deteriorate as a consequence of uterine contractions. Under certain conditions, for example in the sub-optimally oxygenated fetus, the resultant intermittent deterioration in oxygenation of the fetus will modify the fetal heart rate pattern in a manner which enables diagnosis of the condition. Several techniques can be used to induce uterine contractions including nipple stimulation and intravenous administration of dilute oxytocin. Such techniques are described in the document entitled “AOCG practice bulletin—Antepartum fetal surveillance”, from the 30 International Journal of Gynecology & Obstetrics, 2000, 68, pages 175 to 186.
The Nonstress Test (NST) provides a measure of variation in the fetal heart rate since heart rate reactivity is considered to be a good indicator of autonomic function in the healthy fetus. NST is based on the premise that the heart rate of a healthy fetus will undergo temporary accelerations coincident with fetal movement. With the mother in the lateral tilt position, the fetal heart rate trace is typically examined for accelerations which peak at least 15 beats per minute above baseline for a period of 15 seconds, measured baseline to baseline. Furthermore, others have attempted to computerize such analysis, a process described in the document entitled “Computerized evaluation of fetal heart-rate patterns”, by Dawes et al from the Journal of Perinatal Medicine, 1994, 22, pages 491 to 499. Typically the fetal heart rate itself would be determined using Doppler Ultrasound techniques.
A variation on the NST technique employs acoustic stimulation of the fetus to induce fetal heart rate accelerations which has the effect of reducing overall NST testing-time. In this context, acoustic stimulation is applied using a purposely-designed loudspeaker positioned on the maternal abdomen, sometimes referred to as an artificial larynx. This process is described in the document entitled “Nonstress testing with acoustic stimulation and amniotic fluid volume assessment: 5973 tests without unexpected fetal death”, by Clark et al from the American Journal of Obstetrics and Gynecology, 1989, 160, pages 694 to 697. The results of this test are interpreted to determine whether the fetus is reactive or non-reactive according to pre-determined criteria.
An evaluation of fetal movement is also considered a valuable indicator of fetal health. This technique is based on the premise that a decrease in fetal movement is often seen as a precursor to fetal death, sometimes by several days. There are several methods which are currently employed to quantify fetal movements, more specifically: “kick counts” as perceived by the mother; Ultrasound imaging and Doppler ultrasound.
The Biophysical Profile (BPP) comprises a Nonstress test as described previously along with four additional observations of the fetus which are facilitated using real-time Ultrasound Imaging. Consequently, the Biophysical Profile comprises five parts, each of which are assigned a score. These scores are then added to derive a composite score which is compared with a predetermined ‘normal’ score to provide a measure of fetal health. The BPP assessment comprises: NST; observation of fetal breathing movements; observation of fetal body and limb movements; observation of the extension of fetal extremities; and determination of the amniotic fluid volume. This process is described in the document entitled “Fetal biophysical profile scoring: selective use of the nonstress test”, by Manning et al from the American Journal of Obstetrics and Gynecology, 1987, 156, pages 709 to 712.
The modified Biophysical Profile (mBPP) is based on the premise that assessment of the Amniotic fluid volume can be used to evaluate long-term placental function. The ‘Amniotic Fluid Index’ is derived from the sum of measurements of the amniotic fluid pockets in each of the abdominal quadrants. Gauging the Amniotic Fluid Index in combination with a Nonstress test forms the modified Biophysical Profile and provides a measure of general fetal well-being. This process is described in the document entitled “AOCG practice bulletin—Antepartum fetal surveillance”, from the International Journal of Gynecology & Obstetrics, 2000, 68, pages 175 to 186.
Umbilical Artery Doppler Velocimetry employs Ultrasound Imaging to assess the flow velocity in the umbilical artery of a fetus. Primarily, this technique aims to identify the disparity between the flow velocity waveforms observed in the umbilical artery of the normal fetus and those observed in the intrauterine growth restricted fetus. More specifically, in the normal fetus the diastolic flow velocity is relatively high, where in the growth-restricted fetus this rate of flow is attenuated. This process is described in the document entitled “Umbilical artery blood flow characteristics in normal and growth-retarded fetuses”, by Erskine et al, from the British Journal of Obstetrics and Gynecology, 1985, 92, pages 605 to 610.
The first aspect of this invention is concerned with one approach to fetal surveillance, more specifically, the assessment of fetal movements. Current techniques used to quantify fetal movements are:
“kick counts”
Doppler ultrasound
Ultrasound imaging
It has been known for centuries that the mother is capable of perceiving fetal movements, more recently it became clear that the mother's perception of “diminished fetal activity” should be regarded as a worrying sign. As such, Fetal “kick counts” as perceived by the mother are a recognized technique for determining the motility of the fetus. Several counting protocols have been applied in an attempt to quantify these movements, in one of the most popular, the mother lies on her side and counts distinct fetal movements. This process is described in the document entitled “A prospective evaluation of fetal movement screening to reduce the incidence of antepartum fetal death”, by Moore et al from the American Journal of Obstetrics and Gynecology, 1989, 160, pages 1075 to 1080. Perception of ten or more distinct movements over a period of 2 hours is considered reassuring. In another approach, the mother counts fetal movements for one hour three times a week, a process described in the document entitled “Fetal movements as an indicator of fetal well-being” by Neldam from the Danish Medical Bulletin, 1983, 30, pages 274 to 278. In this instance a reassuring count is considered to be one which equals or exceeds the previous count. This technique is simple to implement, but is largely considered a pre-cursor to further fetal assessment in the absence of a reassuring count.
The Doppler ultrasound technique consists of directing a 2 MHz (or other similar frequency) crystal transducer at the fetus on the mother's abdomen. The signal reflected from the fetus is shifted by a small frequency (known as the Doppler shift) which is due to movements of, or within, the fetus. In its customary role Doppler ultrasound is employed to identify pulsations of the fetal heart in order that (after suitable processing) a fetal heart rate (fHR) trace can be produced. However, the Doppler ultrasound technique itself can be employed to identify all movements in its path including fetal body movements, limb movements and breathing movements. In its normal mode of operation as a fHR recorder, the reflected Doppler signal is filtered to remove frequencies which appear outside of the range into which the fetal heart rate will fall. Signals relating to fetal movements however, are generally evident outside of this frequency range. Therefore, in the event that the Doppler signal is filtered to cover a different frequency range i.e. signals which are reflected at other frequencies, it is possible to use the technique to identify fetal movements. Such a system, using an adaptation of a conventional Doppler ultrasound unit, has been developed and is described in the document entitled ‘Neurobehavioral development in the human fetus” by James et al, Fetal Development—A Psychobiological Perspective, 1995, pages 101 to 128. A limitation of the Doppler Ultrasound technique is apparent in single channel Doppler systems which require periodic re-positioning of the transducer to point at the fetus as it moves around the uterus, this requires the intervention of clinically trained staff. In addition to prevent the transducer from moving over the mother's abdomen it is held in place with a belt which can prove to be uncomfortable for the mother. A multi-channel Doppler ultrasound unit has also been described in the document entitled “Fetal heart rate recorder for long-duration use in active full-term pregnant women”, by Shono et al from Obstetrics and Gynecology, 1994, 83, 2, page 301. This consists of six Doppler transducers positioned on the abdomen with each being optimally positioned for the various stances of the mother during her day. However, due to the nature of the ultrasound signal being directed at the fetus then long-term recordings of fHR using ultrasound may be considered to be invasive. Although this invasive nature has not been clinically substantiated, the bulky instrumentation and restraint belts necessary to implement the technique mean that the use of Doppler ultrasound is still limited to short time periods. In summary, Doppler Ultrasound can be used to monitor fetal movements and to produce an “actogram” or “fetal movement profile” which identifies fHR and fetal movement over short time periods.
The development of Ultrasound Imaging proved to be the catalyst to much greater understanding of fetal movements than was previously the case. This technique is non-invasive and enables images of organs and structures within the human body to be displayed on a monitor, illustrating for example the location and size of a structure without the necessity for surgery.
Ultrasound Imaging enables specific fetal movements to be isolated and defined, more particularly: fetal body movements; fetal limb movements; fetal mouthing; fetal eye movements and fetal diaphragmatic movements. Fetal body and limb movements are arguably the most important variables to monitor in an assessment of fetal health. One definition of ‘normal’ fetal activity cites three or more discrete body or limb movements within thirty minutes to be acceptable. Fetal eye and mouthing movements are another commonly observed indication of fetal activity, indeed it has been suggested that mouthing movements are a good discriminator of fetal acidosis. Diaphragmatic movements of the fetus typically refer to fetal breathing but can also include movements which have been described as sighs or hiccups. Definitions as to what is acceptable in this context vary but it is typically quantified as one or more episodes of fetal breathing movements in excess of thirty seconds within a time window of thirty minutes.
Typically, information gathered during Ultrasound Imaging will be presented in the form of an actogram, which displays all of the individual components of fetal movement along with a simultaneously recorded MR trace plotted against a time axis. While Ultrasound Imaging provides detailed information about fetal movement, it is limited to hospital use by the sheer size of the instrumentation itself and the necessity for highly trained clinicians to supervise its operation.
Many years ago it was believed that all motor behavior of the fetus was the result of a known or unknown stimulus. Latterly, with the onset of Ultrasound Imaging enabling the fetus to be observed in its own environment it was demonstrated that specific movement patterns remain recognizable throughout gestation and that they show clear developmental trends, which supports the view that fetal motility is generated spontaneously by the fetal central nervous system and as such is one of the fundamental expressions of early neural activity as described in the document entitled “The emergence of fetal behavior I. Qualitative Aspects” by de Vries et al from Early Human Development, 1982, 7, pages 301 to 322.
The greatest advantage of the fact that the fetus makes body movements could be considered to be the reassurance it provides to the mother. Though the presence of fetal movement alone cannot be considered a substantiation of fetal health, the cessation of these movements during gestation is considered a very worrying sign.
Randomized studies have demonstrated that records of fetal motility kept by the mother to record “kick counts” contribute to a decrease in fetal mortality as described in the document entitled “Fetal movements as an indicator of fetal well-being” by Neldam from the Danish Medical Bulletin, 1983, 30, pages 274 to 278. A more objective means of study is preferable since maternal perception of movements can vary depending on the motility of a given fetus and the degree of movement felt by a given mother.
Moreover, along with recording of fHR, screening for fetal movements is one of the main assessment methods for studying neurobehavioral development of the fetus.
The second aspect of this invention is concerned with a technique to determine fetal presentation and position during gestation.
Fetal presentation describes the orientation of the fetus within the maternal uterus in terms of the part of the fetus which lies at the pelvic brim and is thus positioned to enter the birth canal first. There are five recognized fetal presentations, these presentations, which are illustrated in FIG. 1 are:
Vertex, shown in figure I (a)
Face, shown in FIG. 1 (c)
Brow, shown in FIG. 1 (b)
Breech, shown in figure I (d)
Shoulder, shown in FIGS. 1(e) and (f)
Of these possibilities; Vertex, Face and Brow are also termed Cephalic presentations. A Vertex presentation is identified when the head of the fetus is flexed to leave the crown as the presenting part. Alternatively, when the head is extended, the Face of the fetus presents and when it is neither well flexed nor fully extended, the fetus has a Brow presentation. It is generally accepted that Cephalic presentation of this type is the most common form of fetal presentation, whilst Breech presentation occurs less frequently. A classical Breech presentation is identified when the buttocks of the fetus present first and both the hips and knees of the fetus are flexed. A Breech presentation also defines the circumstance where the hips of the fetus are flexed so that the legs are fully drawn toward the chest, or when the feet or knees present first. A Shoulder presentation occurs when the fetus is in a transverse lie, causing the shoulder, arm or trunk to exit the uterus first.
Fetal position describes the rotational position of the fetus within the uterus for Vertex presentations only. The six fetal positions associated with the Vertex presentation and illustrated in FIG. 2 are: Right Occipitoanterior (ROA), FIG. 2(a); Right Occipitolateral (ROL), FIG. 2(c); Right Occipitoposterior (ROP), FIG. 2(e); Left Occipitoanterior (LOA), FIG. 2(b); Left Occipitolateral (LOL), FIG. 2(d) and Left Occipitoposterior (LOP), FIG. 2(f). Fetal position can be defined more accurately as the relationship between the fetus and six discrete points on the pelvic brim as illustrated in FIG. 3.
During gestation, fetal presentation and position are typically identified using palpation, i.e. manipulation of the maternal abdomen by a clinician or midwife to determine the fetal lie by feel alone. Alternatively, Ultrasound Imaging techniques are frequently employed to provide a more scientific indication of both fetal presentation and position within the maternal abdomen.
Additionally, several publications have highlighted differentiation between abdominal fetal ECG waveforms which appears to be dependent upon the presentation of the fetus. Specifically, the major deflections that can be identified in the abdominal fetal ECG coincident with the strike of the fetal heart (analogous to the QRS complex in adult ECG waveforms) in Breech presentation were more or less the inverse of those in a Vertex presentation. This observation is described in several documents, including that entitled “The fetal ECG throughout the second half of gestation”, by Oostendorp et al from Clinical Physics and Physiological Measurement, 1989, 10, 2, pages 147 to 160 and also in the document entitled “The Fetal Electrocardiogram V. Comparison of lead systems”, by Roche et al from the American Journal of Obstetrics and Gynecology, 1965, 92, 8, pages 1149 to 1159.
Up to 14% of babies are in a breech presentation until the 29th to 32nd week of gestation. Many, but not all, of these babies change to a vertex presentation preceding birth. It is important that the fetus is in a Vertex or other Cephalic presentation pre-delivery to minimize the risk of asphyxia caused by cord strangulation, and to simplify delivery. Furthermore, considering fetal position, Occipitoanterior positions are often considered more favorable than Occipitoposterior positions. This is a consequence of the back of the fetus conforming with the concavity of the maternal abdominal wall hence allowing it to flex more readily. As a corollary of this, there is also a tendency for the head to flex, presenting a smaller diameter to the pelvic brim.